FI90792C - Method and apparatus for purifying a fiber suspension - Google Patents

Description

90792

Method and apparatus for purifying a fiber suspension For use in the preparation of a fiber suspension

The invention relates to a pressure screen for cleaning a fiber suspension and to a method for cleaning a fiber suspension in a pressure screen. The invention also relates to the use of such a pressure screen in a short cycle of a paper machine. The invention relates mainly to the removal of impurities from heat suspensions used in the pulp and paper industries. The invention discloses the further development of commonly used pressure screens which minimize the loss of good material with the reject so that the cleaning process is made more efficient, compact and energy efficient.

In a pressure screen of a known type, disclosed, inter alia, in U.S. Pat. No. 3,363,759, the fiber suspension is fed into a space bounded on at least one side by a screen plate through which the fiber suspension is pressed and which prevents the bundles and other larger bodies from entering the screen. Due to the filtration mechanism, some of the good fibers stop in front of the screen, forming a thickening non-fibrous mat that blocks the flow. The fibers accumulating on the screen plate are resuspended in said US patent by means of turbulence-causing protrusions on the surface of the rotor.

The temporary retention of the fibers on the screen plate also results in the liquid component of the suspension passing through the sieve more than the fibers, resulting in a gradual thickening of the suspension to be purified in the screening zone. This, in turn, gradually impairs the performance of the screening until the screening has to be stopped to prevent excessive thickening and consequent clogging of the screen.

The removal of the screen-impermeable fraction, the reject, from the screen also requires sufficient volume flow per se in order to keep the flow rate in the reject tubes sufficiently high and to prevent solid material separation and consequent clogging of the piping.

Thus, due to these phenomena, a considerable proportion of the good fiber, typically about 5-30%, has to be discarded in a conventional pressure screen. To recover these fibers, the reject is re-diluted and purified in a second cleaning step in a second screen, which may in turn be cleaned in a third step, etc. Usually the reject from the last 2 cleaning steps is passed to a different type of open screen where good fibers are washed back from the jet. process and from which the reject can be removed at high consistency.

Multi-stage cleaning is obviously disadvantageous in terms of space requirements, investments, energy consumption, cleanliness and otherwise manageability. Washing and returning the reject from the final step to the process cycle through open containers is unstable and causes secondary effects such as splashing and dirt formation, mucus absorption into water, and so on.

As disclosed, for example, in U.S. Pat. No. 3,437,204, the dilution of the suspension to be sieved is known in the art, the dilution being e.g. performed through a rotor associated with the screen. However, in the prior art, it has been found disadvantageous to feed dilution water through a rotor in a pressure screen, since the water thus fed has been found to mix poorly with the suspension to be screened and to form an anti-screening liquid layer on the screen surface. Thus, said U.S. patent discloses diluting the suspension through tubes passing through a fixed screen plate.

EP 0233517 A2 discloses a pressure screen in which the suspension is diluted through a recess in the screen plate. According to the patent publication, such a screen can be used for washing recycled pulp. However, such strainers are not suitable for cleaning the paper machine feed because the feed of the dilution water is too local, causing an uneven fiber content in the accept. In addition, the dilution water added in this way drains quickly through the sieve plate, leading to a poor dilution efficiency and the need for too much dilution water.

One problem when using fibrous process water as a diluent is the formation of hot bundles and the formation of slime on the surfaces of pipelines. However, it is known that the piping remains self-clean if the flow sweeps the wall of the pipe at a sufficient speed, i.e. at least about 3 m / s, and there are no sharp angles or dead spots in the piping slowing down the flow. It is also known that in such situations turbulence prevents the formation of fiber bundles.

The object of the present invention is to avoid the limitations of the prior art by feeding dilution water in smaller partial flows through the rotating part of the pressure screen, so that the previously problematic kinetic energy of the dilution water is utilized to create a useful turbulence and a condensed heating layer.

II

90792 3 to resuspend. It is also an object of the invention to avoid clogging of nozzles and channels, which has previously been a problem.

According to the invention, there is provided a process in which continuous dilution counteracts the narrowing of the fibrous suspension to be screened and which allows almost complete removal of acceptable fibers from the reject prior to removal from the screen. This allows for efficient purification of the fiber suspension in a single step and provides a more simple and inexpensive process.

Particular features of the invention will become apparent from the appended claims and the following description. Accordingly, the invention relates to a pressure screen for screening a fiber suspension, in which a cylindrical perforated screen drum has a screen housing and a rotor to introduce dilution water into the suspension. The pressure screen according to the invention is characterized in that the rotor has a dilution water piping which communicates with a number of dilution water nozzles rotating with the rotor and opening close to the screen surface of the screen drum.

In the method according to the invention, the suspension is diluted during the cleaning process inside the screen drum via a rotating rotor, the method being characterized in that the dilution water is led in a locally directed turbulent flow close to the screen surface.

The pressure screen according to the invention is particularly well suited for use in a short circuit of a paper machine immediately before the headbox, whereby zero water is introduced into the dilution water nozzles, which is circulated from the web-forming wire as an airless stream directly to the pressure screen.

The invention will now be described in more detail with reference to the accompanying figures, in which Fig. 1 shows a schematic sectional view of a preferred embodiment of a screen according to the invention, Fig. 2 shows a turbulence generator and 3b show dilution water nozzles of a straight projection of a rotor jacket.

Figure 1 shows generally a pressure screen according to the invention having a substantially cylindrical screen housing 12 and a substantially cylindrical, perforated screen drum 16 therein, the holes of which are designed to pass through the acceptor of the fibrous suspension to be screened. Inside the sieve drum 16, at a distance from its wall, there is a rotor 14 concentric therewith. At the inlet end of the sieve there is a heat suspension feed connection 10 and an injection chamber 11 bounded by the bottom 12 'of the screen housing 12 and the bottom 14' of the rotor 14, respectively. An annular screening zone 15 is formed between the cylindrical silicon drum 16 and the jacket 14 "of the rotor 14. Outside the screen drum 16, an acceptance chamber 17 is formed between the drum 16 and its screen 12", respectively.

In a preferred embodiment of the invention, the jacket 12 "of the screen housing 12 is weakly conical and likewise the jacket 14" of the rotor 14 is weakly conical so that their diameters at the inlet end are smaller than at the outlet end. However, the ratio drum 16 between them is preferably a substantially cylindrical body. Due to the conical shape of the jackets, the total volume of the liquid chambers of the sieve increases towards the outlet end so that the increasing amount of liquid due to dilution can flow in the sieve. Due to the cylindrical shape of the screen drum 16, the screening zone 15 is in the vicinity of the inlet end than in the outlet end and, correspondingly, the acceptance chamber 17 is inferior in the outlet end than in the inlet end.

The acceptor chamber 17 is connected to the acceptor outlet pipe or outlets 24 opposite the injection chamber 11 and the reject outlet pipe 23 opens to the screening zone 15. In the preferred embodiment of the invention there are several which distribute the mass evenly in the headbox.

A plurality of dilution water tubes 22 run on the shaft 28 of the rotor 14, which enter the rotor 14 and terminate in dilution water nozzles 20 on the surface of the screen drum 16, which are numerous in number along the surface of the rotor 14. The distance between the rotor 14 and the screen drum 16 is relatively small so that the nozzles 20 open relatively close to the surface of the screen drum 16.

. In use, the fiber suspension is fed to the screen through an inlet connection to the injection chamber 11, from where it continues to the screening zone 15. The feed pressure and centrifugal force cause flow through the screen drum 16 into the also a statistical proportion of acceptable fibers.

Since the screen surface retains both the material to be removed and also some of the acceptable fibers, relatively more liquid passes than the solid material through the screen drum 16. The dry matter content of the suspension in the screening zone 15 then tends to increase. The increase in dry matter content is counteracted by introducing dilution water into the screening zone. According to the invention, the dilution water is fed through pipes 22 running in the rotor 14 to nozzles 20 on the surface of the rotor. .

The flow can also be facilitated by generating turbulence and countercurrent pressure pulses on the screen surface by means of mechanical turbulence generators 18. The turbulence generators 18 may be, for example, protrusions on the surface of the rotor or separate vanes.

Figures 2 and 2a show how the dilution water according to the invention is introduced into the screening zone 15 in such a way that it is introduced in a well-defined, rapid flow near the screen surface of the screen drum 16, whereby the kinetic energy of the water is released into turbulence. The turbu-lentti flow also entrains part of the fibrous suspension to be filtered. Thus, a turbulence zone 21 is created on the surface of the screen drum 16 in connection with each dilution water nozzle 20. This turbulence facilitates the mixing of the dilution water into the suspension as well as the mobilization of the fibers felt on the screen surface and also their resuspension in the diluted suspension.

In the solution according to Figures 2 and 2a, the nozzle 20 is placed in connection with the mechanical turbulence generator 18 so that a strong local turbulence region 21 is achieved by their interaction, which sweeps over the screen surface and scatters the accumulated fiber layer. The nozzles 20 are preferably mounted so as to open towards the direction of rotation of the surface of the rotor 14 by spraying water against the screen surface. If the rotor 14 is conical and the screen drum 16 is cylindrical, it is advantageous to design the turbulence generators 18 in such a way that the nozzles 20 are made very close to the screen surface, despite the larger distance between the rotor and the screen. The number of dilution water nozzles 20 is chosen to be large enough, typically 5 to 100, preferably 10 to 50, to clean the entire screen surface and so that the local dilution of the suspension caused by the separate nozzles is small enough to equilibrate before the acceptor discharges from the screen to the next destination, which is typically the headbox of a paper machine. Preferably, the dilution water nozzles 20 are positioned so that at least one area of turbulence 21 sweeps over each point of the screen surface with each revolution of the rotor 14. Examples of such a nozzle arrangement are shown in Figures 3a and 3b, which show a straight rotor casing. The arrangement of the nozzles 20 is selected in proportion to the flows inside the screen so that interference between the local dilutions they cause is avoided. Thus, there are many different embodiments of the invention corresponding to Figures 3a and 3b.

Figure 1 shows an embodiment of the device according to the invention, in which each nozzle 20 has its own branchless suction connection 22, which avoids dead spots and the disturbances caused by fiber bundles and fouling, such as clogging of pipes and nozzles or fouling of the fiber suspension. The pipes 22 are dimensioned in relation to the designed flow rate so as to provide a flow rate sufficient for continuous cleaning of the inner surfaces, preferably at least about 3 m / s. In particular, care must be taken to ensure that the liquid is discharged through the nozzle 20 at a sufficient rate to produce substantial turbulence and also that the nozzle 20 remains clean.

If the number of nozzles 20 is very large, it is difficult to install a separate tube 22 for each nozzle in the rotor 14. In this case, the tubes of nozzle groups at the same level or even at adjacent levels can be connected so that distribution to separate nozzles takes place inside the rotor. The supply of dilution water to all nozzles can also take place via a single branch pipe, but it is advantageous to use several separate dilution water pipes in the rotor in order to achieve a uniform and sufficiently fast flow. When a branch or branches are included in the dilution water pipes, they are formed according to the prior art 90792 7 in such a way that the distribution connection remains in such a turbulent state due to the flow that it is avoided from fouling and formation of fiber bundles.

The dilution water is preferably fed to the intake connections 22 via a rotary joint 27 arranged in the shaft 28 of the rotor 14. In special cases, if it is desired to control the dilution flow, the dilution water connections can be grouped into groups, each with its own separately adjustable flow rate intake.

Figures 4 and 4a show a second embodiment of the core of turbulence generators 18 and dilution water nozzles 20 according to the invention, in which hollow arms 35 carrying separate vanes 34 are connected to the rotor 14. In this embodiment, dilution water pipes 22 pass through the arms 35 and the vanes 34 to zone 15 in the vicinity of the screen surface.

The dilution water nozzles 20 can be positioned at different points and directions on the vane 34 and also on the arm 35. In a preferred embodiment is the solution shown in Figure 4, where the nozzles 20 are positioned at the leading edge of the vane 34 in front of the vane 34 to induce induced turbulence. In this embodiment, each vane 34 is provided with a plurality of forward nozzles 20, which are preferably spaced 2 to 10 cm apart. The location of the nozzles is preferably stepped between the vanes so that each part of the screen surface of the screen drum 16 is swept by at least one turbulence zone 21 with each revolution of the rotor 14.

The suspension to be purified by the process of the invention is maintained by continuous dilution at a consistency suitable for efficient screening until the majority of acceptable fibers have passed through the screening zone 15 of the screen drum 16. Since the reject held by the screen drum 16 then tends to thicken, the final dilution is carried out through the reject dilution nozzles 25 located at the end of the screening zone, whereby the consistency of the reject is reduced to a level where it can be well removed through the reject connection 23 without clogging. In this embodiment, the reject dilution water is introduced into one of the tubes 26 and the nozzles 25 of the separate rotor 27 arranged on the shaft 28 and further into the nozzles 25.

Figure 5 shows a paper web forming process in which the advantages of the one-step pressure screen according to the invention are particularly well utilized. In the process shown, which is detailed in the co-pending patent application (POM / PROCESS), the pulp is fed in a controlled flow of 8 to 5% as a suspension from the pulp production 124 into a short circuit. The pulp is first diluted to a consistency of about 0.5 to 1.5%, after which it is passed to a separator 122. The centrifugal separator disclosed in the co-pending patent application (POM / CLEANER), which operates in one step without return of the reject. The separator uses air-free recirculated zero water for pulp dilution and reject washing. The purified pulp is then fed to a pressure screen 121 according to the invention, in which recirculated zero water is also used airlessly as dilution water.

The pulp from the pressure screen is introduced into the headbox 100 of the paper machine, which can advantageously take place via a special manifold 125. The manifold 125 consists of a relatively large number of acceptor tubes of the screen 121, which are arranged so that they are all substantially identical, as well as such that the number of possible steep curves and the curvature of the tubes are substantially identical. With this arrangement, an even distribution of the pulp over the entire width of the paper machine can be guaranteed.

From the headbox, 100 masses are fed to the web forming area. At the beginning of the forming area, most of the water contained in the pulp suspension seeps into separate boxes 111-120 under the wire, of which preferably at least the boxes at the beginning are hydraulically determined, water-filled boxes. Zero water is returned directly to the main process flow by means of separate pumps without open vessels. In order to recirculate the zero water substantially substantially airless, deaeration pumps are used as required, which are preferably gas separation pumps according to the co-pending patent application (POM / PUMP).

According to the preferred process shown, the recirculated zero water is discharged as separate streams to the various dilution sites of the short circuit so that the dilution water required by the pressure screen according to the invention and the centrifugal separator preceding it in the pulp flows directly into the pulp mains without piping branches. The process also ensures that the zero water that first seeps through the wire, which contains most of the fibrous material permeated by the wire, returns as close to the headbox as possible.

The single-stage operation of the screen according to the invention without return of the reject and without water and suspension circulating systems substantially speeds up the achievement of a new equilibrium state in connection with the change of paper quality and thus considerably reduces the amount of paper wasted during quality change.

Claims (16)

90792 A pressure screen for screening a fiber suspension, wherein a substantially cylindrical perforated screen drum (16) has a screen housing (12) and a rotor (14) around it, thereby forming a reject retaining zone (15) between the screen drum (16) and the rotor (14). and an acceptor receiving acceptor chamber (17) is formed between the rotor (14) and the screen housing (12), and an acceptor outlet tube (24) is connected thereto, and the rotor (14) has means for introducing dilution water into the suspension, characterized in that the rotor (14) has dilution water -to (22), which leads directly to a plurality of rotating dilution water nozzles (20) opening close to the screen surface of the screen drum (16). Pressure screen according to Claim 1, characterized in that there are 5 to 100 dilution water nozzles (20). Pressure screen according to Claim 1 or 2, characterized in that the nozzles (20) are designed to supply the dilution water locally at a high speed close to the screen surface. Pressure screen according to one of the preceding claims, characterized in that the nozzles (20) are located on the surface of a cylindrical rotor (14) concentric with the screen drum (16). Pressure screen according to Claim 1, 2 or 3, characterized in that the nozzles (20) are arranged on the vanes (34) forming part of the rotor (14), each vane (34) having a plurality of nozzles (20). Pressure screen according to one of the preceding claims, characterized in that the nozzles (20) open in the direction of rotation of the rotor (14). Pressure screen according to one of the preceding claims, characterized in that the nozzles (20) are mounted in connection with turbulence-generating elements (18) on the surface of the rotor (14). Pressure screen according to one of the preceding claims, characterized in that a separate dilution water pipe (22) running through the axis of the rotor (14) leads to separate nozzles (20) or groups of nozzles. Pressure screen according to one of the preceding claims 1 to 8, characterized in that it is designed to receive an increase in the amount of liquid due to dilution, the screen housing (12) and the rotor (14) being conical, the screen drum (16) being essentially cylindrical so that the screening zone (15) is wider at the inlet end of the screen than at the outlet end and the accept chamber (17) is wider at the outlet end than at the inlet end. Pressure screen according to one of the preceding claims, characterized in that a reject dilution water pipe (26) running through the axis of the rotor (14) leads to the outlet end of the screen drum (16). 11. A method of cleaning a fibrous suspension in a pressure screen, wherein the suspension is diluted during a cleaning process within a screen drum through a rotating rotor, characterized in that the dilution water is passed in the piping directly as a locally directed turbulence stream near the screen surface. Method according to Claim 11, characterized in that the dilution water is passed at a high speed through a plurality of nozzles which open close to the screen surface. Method according to Claim 11 or 12, characterized in that the dilution water is led in separate tubes in the rotor with a flow of at least 3 m / s. Use of a pressure screen according to any one of claims 1 to 10 in a short cycle of a paper machine immediately before the headbox (100), wherein zero water is fed to the dilution water nozzles of the pressure screen (121) and recirculated directly from the web forming wire to the pressure screen (121). Use according to Claim 14, characterized in that zero water recirculated from the web-forming wire in the form of airless separate streams is supplied to the separate nozzles (20) or groups of nozzles via separate dilution water pipes (22). Use according to Claim 14 or 15, characterized in that the acceptor obtained from the pressure strainer (121) is led to the headbox (100) via a manifold (125) with a large number of acceptor tubes in which the flow path and resistance are substantially identical. 90792